1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to an apparatus for and a method of adjusting the partial coherence of the light energy in an imaging system, such as a step-and-scan tool used in photolithography.
2. Discussion of Related Art
The performance of a microprocessor comprising transistors is primarily determined by the channel lengths of the transistors. The channel lengths are highly dependent on the widths of the gates in the transistors. Photolithography is the process by which a pattern of gates can be transferred from a reticle to a layer of photoresist on a wafer during the fabrication of a microprocessor. The reticle is a photomask which has a pattern of gates extracted from a layout of a microprocessor. Photoresist is a radiation-sensitive material. Radiation is the transfer of energy as waves or particles. The application of radiation in the form of light, electron, or ion energy to form a latent image in the photoresist is called exposure. Exposure is done in an imaging tool, such as a step-and-repeat tool or a step-and-scan tool. After exposure, a develop process selectively removes portions of the photoresist layer corresponding to the latent image. The pattern realized in the photoresist is, in turn, replicated through an etch process in the conductive material forming the gate.
The yield of microprocessors fabricated on a wafer is affected by the variability in critical dimension (CD) of the gate widths. Gate CD is influenced by a variety of factors, with some being systematic and others being random. One type of effect derives from the interaction of photolithography with wafer topography, thin film thickness, and substrate reflectivity. A second type of effect comes from non-uniformity of CD across the reticle. A third type of effect involves imprecision and inaccuracy of the imaging tool in leveling, focusing, or exposing the wafer. However, aberrations in the optics of the imaging tool have become an increasingly important determinant of gate CD variability as dimensions in the transistors shrink below 130 nanometers.
A gate layer is typically printed on a wafer with a step-and-scan type of imaging tool so as to obtain images with sufficiently high fidelity and accurate placement. A wafer is partitioned into identical small areas called fields that are appropriately aligned and sequentially exposed by stepping and scanning. Stepping refers to shifting to a predetermined location of the wafer in order to expose a desired field with the imaging tool. Scanning refers to synchronously moving a reticle and a wafer with respect to the projection optics in order to “shine” light energy through a slit and “paint” the light energy across a reticle onto a field on the wafer.
CD variability along the scan direction is inherently reduced since scanning the slit across the field has a smoothing effect. However, CD variability along the slit direction may be caused by aberrations in the optics or by non-uniform distribution of dose or partial coherence.
Dose is the amount of light energy per unit area delivered to the wafer plane. Non-uniform dose in the direction along the slit may be compensated by adjusting the slit width as a function of the slit length. The slit width may be changed by using an aperture with one fixed edge and one articulated edge.
Partial coherence is the numerical aperture (NA) of the illumination optics divided by the NA of the projection optics. The NA is a measure of the divergence angle of the light energy. NA may be varied by changing the size of an aperture stop at a pupil plane of the condenser or relay lens system.
The optical efficiency of an imaging tool may be enhanced by using a non-tool specific array at or near a conjugate plane to isotropically expand light into a region slightly larger than the desired illuminator NA. An aperture stop may be used to “clean up” the light as needed.
As gate CD on a microprocessor continually shrinks, the within-field CD variability consumes an increasingly larger portion of the overall CD error budget. The within-field CD variability encompasses isolated-dense (iso-dense) bias and horizontal-vertical (H-V) bias. Iso-dense bias refers to CD variability caused by proximity to other features. H-V bias refers to CD variability caused by aberrations in the optics or by non-uniformity in partial coherence between two orthogonal directions, in particular, the horizontal direction and the vertical direction.
FIG. 1 shows a tool-specific array 110 that minimizes H-V bias in the direction along the slit. Customization is based on the aberration signature of a particular projection optic. The ellipses represent the illuminator NA into which the tool-specific array 110 sends light energy. The tool-specific array 110 comprises optical elements 120 that maintain a constant partial coherence ellipticity in a first dimension 130 while varying partial coherence ellipticity in a second dimension 140. The first dimension 130 and the second dimension 140 are orthogonal to each other. When a step-and-scan type of imaging tool is used to pattern the gate layer in a microprocessor, the first dimension 130 corresponds to a direction along the scan while the second dimension 140 corresponds to a direction along the slit.
Unfortunately, a tool-specific array 110 incurs significant cost and requires a long time to build. Furthermore, the customization must be repeated whenever major changes are detected in the aberration signature of the imaging tool or the sensitivity of the product.
Thus, what is needed is an apparatus for and a method of adjusting the partial coherence of the light energy in an imaging system, such as a step-and-scan tool used in photolithography.